1. Field of the Invention
The present invention relates to a magnetic disk drive and, more particularly, to a magnetic disk drive capable of servo control operation on the basis of servo control data stored previously in a magnetic disk.
2. Description of the Prior Art
A hard disk drive, which is one type of magnetic disk drive, includes a magnetic disk, consisting of a hard, metallic disk (typically referred to as a hard disk) upon which is formed a magnetic recording layer, and a magnetic head for recording information onto reproducing information from the magnetic disk. The magnetic recording layer is divided into a plurality of storage locations which are arranged in number of concentric tracks. Because the tracks are closely spaced in high track density hard disks, servocontrol data is typically provided on the magnetic recording layer to help position the magnetic head over a target track during a seek operation in order to perform recording or reproducing operations on the target track. As shown in FIG. 3, a magnetic disk 1 has a flotation security region 2 formed in a magnetic layer forming area on the surface thereof. The flotation security zone 2 has, in a sequential arrangement from the inner circumference outward, an inner guard band 3, a contact start/stop zone (hereinafter, referred to as "CSS zone") 4 in which no data is recorded and against which a magnetic head is parked when the magnetic disk is not spinning a data zone 5 in which data is recorded, and an outer guard band 6. A 0th track is formed on the outer circumference of the data zone 5 and a 614th track, for instance, is formed on the inner circumference of the data zone 5. The flotation security zone 2 is divided at angular intervals into a plurality of sectors. A servo zone 8 in which servocontrol data is recorded is formed at the leading end of each sector. The magnetic head reads the servocontrol data recorded in the servo zones 8 during a seek operation to position the magnetic head over a predetermined track.
Referring to FIG. 4, each servo zone 8 is divided into two subzones, namely, a leading zone 9 and a trailing zone 10. Servo data 9a is recorded in the leading zone 9, which is on the leading side of trailing zone 10 with respect to the direction of rotation of the magnetic disk indicated by the arrow R in FIG. 3, and servocontrol data 10a is recorded in the trailing zone 10, which is on the trailing side of leading zone 9 with respect to the direction of rotation of the magnetic disk. The servocontrol data 9a and 10a are recorded alternately in the leading zone 9 and the trailing zone 10 respectively on the opposite sides of the center lines 11a of the corresponding recording tracks 11 in a zigzag arrangement. The servocontrol data 9a and 10a are signals of the same frequency. Signals 12 on the leading side of the servocontrol data 9a has a known AGC (automatic gain control) pattern.
Sometimes, this prior art magnetic disk drive fails to read the servocontrol data recorded on the magnetic disk during a seek operation. If the magnetic disk drive fails to read the servocontrol data, a magnetic head shifting device continues to move (shift) a magnetic head unit on the basis of the servocontrol data read in a preceding magnetic head shifting cycle, so that correct servocontrol operation cannot be achieved. If the failure to read the servocontrol data is temporary and attributable to instantaneous noise, and the servocontrol data can be read correctly immediately after the failure, and no significant problem will occur. However, if the failure to read the servocontrol data occurs repeatedly, then servocontrol of the magnetic head unit is impossible, that is, the magnetic head unit is not decelerated properly as the head approaches the desired track, and as a result, the magnetic head unit collides hard against a stopper. This collision causes the magnetic head to be shaken and can cause damage the magnetic disk.